The present invention relates to processes for treatment of offgas resulting from production of a metal chloride, such as aluminum chloride, by chlorination of the corresponding metal oxide or other metal-containing material. Such offgas typically contains Cl.sub.2, CO, CO.sub.2, HCl, COCl.sub.2, COS, SO.sub.2, N.sub.2, SiCl.sub.4 and metal chlorides. The present invention relates to a process for treatment of the offgas to remove all of the aforementioned components except CO.sub.2 and N.sub.2, which are released to the atmosphere after treatment of the offgas by the process of the invention.
A process for treatment of aluminum chloride offgas is disclosed in our co-pending U.S. application Ser. No. 959,927, filed Dec. 12, 1977. However, the preferred process disclosed therein suffers from the disadvantage of requiring adsorption of SiCl.sub.4 and metal chlorides onto dry carbon adsorbers prior to hydrolysis of COCl.sub.2 in a bed of activated carbon, and passing a stripping gas through the dry carbon adsorbers to remove the SiCl.sub.4 and metal chlorides and to regenerate the dry carbon adsorbers. This preferred process has been found to be excessively costly because of the expense of heat and gases needed to regenerate the carbon adsorbers, the need to replace spent activated carbon in the adsorbers, and the production of excess weak aqueous HCl during hydrolysis of COCl.sub.2. It is a principal object of the present invention to eliminate the foregoing disadvantages of the preferred process described in co-pending U.S. application Ser. No. 859,927.
Various methods for treatment of individual components of offgas from aluminum chloride production are taught in the prior art, but combination of such prior art methods would result in a method which is less than completely suitable for treatment of offgas from aluminum chloride production. For example, Richert et al U.S. Pat. No. 3,314,753 teaches removal of phosgene from a waste gas containing the same together with organic materials. Phosgene is decomposed by reaction with water vapor in the presence of activated carbon at a temperature of at least 120.degree.-400.degree. C., which is above the dew point of the water-phosgene mixture. Richert et al do not mention the presence of metal chlorides or SiCl.sub.4, their removal from a gas stream or the disposal of such metal chlorides after removal. While activated carbon initially does catalyze the hydrolysis of SiCl.sub.4 and metal chlorides as well as phosgene, it has been found that using only a single bed of activated carbon to catalyze such hydrolysis is not satisfactory because deposition of SiO.sub.2 and metal oxides on the carbon resulting from hydrolysis of SiCl.sub.4 and metal chlorides severely diminishes the effectiveness of catalysis with respect to phosgene hydrolysis.
A single bed of activated alumina is likewise not entirely suitable for treatment of aluminum chloride production offgas even though activated alumina initially catalyzes hydrolysis of SiCl.sub.4, metal chlorides and phosgene. After significant quantities of SiO.sub.2 and metal oxides are deposited on activated alumina catalyst, the catalyst becomes ineffective with respect to phosgene hydrolysis. Frevel et al U.S. Pat. No. 3,376,113 discloses activated alumina as a catalyst for hydrolysis of phosgene at temperatures of 95.degree.-190.degree. C.
Halogen-containing gases and vapors, such as phosgene (COCl.sub.2), HCl, silicon tetrachloride, titanium tetrachloride, iron chloride, and aluminum chloride, as well as Cl.sub.2 present in the offgas from an aluminum chloride reactor, must be removed from the gas before it is emitted to the atmosphere even when present at low levels. While it is well known, for example, that silicon tetrachloride will react with water to form a gelatinous precipitate commonly referred to as silica gel, this reaction is not desired, and, in fact, it is to be avoided because of the plugging of the pipelines and vessels which can occur upon formation of this gelatinous precipitate.
Brzozowski in U.S. Pat. No. 3,615,163 contacts titanium tetrachloride in waste gas with steam to avoid formation of a gelatinous precipitate. However, the patentee fails to mention the presence of any solid particles in the vessel in which reaction takes place. Similarly, in Low U.S. Pat. No. 1,451,399, silicon chloride is contacted with steam in an open container. There is no mention of catalysis by solid particles in the Brzozowski and Low patents.
In U.S. Pat. No. 3,980,755, Black et al teach the removal of chlorinated organics (ethers) by air using silica gel or activated alumina. This reference fail to teach or suggest that hydrolysis of SiCl.sub.4 or metal chlorides would be enhanced by carrying out the reaction in the presence of solid particles.
Krchma in U.S. Pat. No. 2,682,930 teaches removal of titanium tetrachloride in a gas by adsorption onto a bed of activated carbon, activated alumina or activated silica in the absence of water, followed by either heating the bed or stripping it with an inert gas such as hot gaseous titanium tetrachloride or chlorine. There is no mention of conversion of the titanium tetrachloride to titanium oxide by hydrolysis and therefore no suggestion that such hydrolysis would be enhanced by being carried out in the presence of solid particles.
Low U.S. Pat. No. 1,451,399 indicates that if silicon chloride is contacted with a jet of steam at a temperature above the dehydration temperature of silicic acid, the silicon chloride will be hydrolyzed to form silica as well as hydrogen chloride by the reaction: EQU SiCl.sub.4 +2H.sub.2 O=4HCl+SiO.sub.2
However, in actual practice, it has been found that this conversion at ordinary steam pressures and at a temperature of about 100.degree. C. results in low reaction rates.
R. F. Hudson in "The Vapor Phase Hydrolysis of Non-Metallic Chlorides", published in Volume 11 of the International Congress of Pure Applied Chemistry London Proceedings in 1947, indicated that this reaction must be carried out at much higher temperatures. He reports that Daubree noted that silicon tetrachloride and water vapor react at red heat in the presence of oxygen to give highly crystallized silica. Hudson then states that as oxygen and silicon tetrachloride do not react until higher temperatures are employed, this indicates a vapor phase hydrolysis at temperatures on the order of 700.degree. C. Hudson then experimentally verified this by reporting experiments conducted in the temperature range of 25.degree.-100.degree. C. wherein no vapor phase hydrolysis of the silicon tetrachloride apparently occurred, at least without the presence of a small deposit of what he termed as "silicon". Hudson then states that he did observe reaction to obtain highly crystallized silica at a temperature of 400.degree. C.
It is also known to produce metal oxides from metal halides in fluidized beds. For example, Hughes et al U.S. Pat. Nos. 3,043,657; 3,043,659 and 3,043,660 are addressed to the production of metal oxides such as titanium dioxide or silicon dioxide by reacting the corresponding chloride with oxygen or air in a fluidized bed at temperatures of 500.degree. C. or higher. Van Weert U.S. Pat. No. 3,642,441 reacts metal chlorides with steam or water vapors in a fluidized bed. However, again the bed is operated at an elevated temperature of 700.degree. C. or higher by the combustion of a gaseous hydrocarbon such as propane in the fluidized bed.
While the reactions reported in the Low and Hughes et al patents and in the Hudson article can be used for disposal of metal chlorides, such as TiCl.sub.4 and SiCl.sub.4, without formation of undersirable gelatinous products, the high temperatures proposed in the references make such processes economically unattractive. It must be recognized that these references were concerned with production of oxides rather than treatment of offgas usually containing only a minor or trace amount of metal chlorides. The present invention differs from these prior art processes by providing a process for treatment of offgas from aluminum chloride production by hydrolysis of metal chlorides without using excessive heat.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification, taken in conjunction with the drawing.